Since a separation technology using a hollow fiber membrane has advantages such as the miniaturization of a device, it is widely used in various fields such as water treatment fields such as water purification, drinking water production, industrial water production and waste water treatment, food industry fields, and pharmaceutical product production fields.
Further improvements in permeation performance, fractionation characteristic, etc. are required for hollow fiber membranes used in such a separation technology. Specifically, if the permeation performance of a hollow fiber membrane is enhanced, a necessary membrane area becomes smaller and a device for realizing the separation technology using the hollow fiber membrane can be further miniaturized. This is advantageous in terms of cost since facility cost and membrane exchange cost can be reduced. Further, the hollow fiber membrane has advantages such as the widening of targets to be removed if the fractionation characteristic thereof can be enhanced.
However, a separation membrane such as a hollow fiber membrane is generally such that fractionation characteristic is reduced if permeation performance is enhanced and permeation performance is reduced if fractionation characteristic is enhanced, i.e. permeation performance and fractionation characteristic tend to be in a so-called trade-off relationship. Thus, it is difficult to improve both permeation performance and fractionation characteristic of the hollow fiber membrane.
On the other hand, a separation membrane using a fluorine-based material such as a vinylidene fluoride-based resin has attracted attention due to high chemical durability, high physical durability and the like. Examples of the separation membrane using such a fluorine-based material include hollow fiber membranes described in patent literature 1 to 3.
Patent literature 1 describes a fluorine-based hollow fiber membrane including a filter area of a sponge structure having pores of an average diameter of 0.01 μm to 0.5 μm, a support area of a sponge structure having pores of an average diameter of 0.5 μm to 5 μm and a backwash area of a sponge structure having pores of an average diameter of 2 μm to 10 μm, wherein the filter area, the support area and the backwash area are successively formed from an outer surface toward an inner surface.
According to patent literature 1, it is disclosed that excellent backwash performance and filter performance can be exhibited while outstanding mechanical strength is possessed.
Further, patent literature 2 describes a porous membrane production method for producing a porous membrane by a nonsolvent induced phase separation method by discharging a membrane forming solution containing at least a polyvinylidene fluoride resin and a solvent and bring the membrane forming solution into contact with a coagulation liquid containing at least a nonsolvent. It is described in patent literature 2 that, in this production method, a discharge temperature of the membrane forming solution is not lower than a melting point of the polyvinylidene fluoride resin and below a decomposition temperature of the polyvinylidene fluoride resin and a temperature of the coagulation liquid is higher than a porous structure formation start temperature of the membrane forming solution.
According to patent literature 2, it is disclosed that a porous membrane having not only excellent water permeability, excellent performance in blocking micropathogens and high chemical resistance, but also capable of stably and sufficiently removing micropathogens for a long period of time can be produced.
Further, patent literature 3 describes a fluororesin-based polymer separation membrane having a three-dimensional network structure and a spherical structure, the three-dimensional network structure containing at least one kind of a hydrophilic polymer selected from cellulose ester, aliphatic vinyl ester, vinylpyrrolidone, ethylene oxide and propylene oxide.
According to patent literature 3, it is disclosed that various performances such as separation performance, water permeation performance, chemical strength (chemical resistance), physical strength and stain resistance can be enhanced.